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I have noticed that when I have lemon juice on my fingers and I touch a polystyrene foam cup (I will refrain from using the term 'styrofoam'), the cup begins to break down, i.e., melt. I can leave fingerprints on the cup in this manner. What is it about lemon juice and foam cups? What is happening, other than what I see; the cup melting.

I have never tried replicating this using other citric acids, such as an orange. So, I don't know yet if it's just lemon juice that will do this.

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It is not any acid in the juice, but limonene, the natural, fragrant, oil found in citrus fruit, and similar terpenes that are good organic solvents.

In fact, these oils are sold for the purpose of cleaning. Note, though limonene itself is considered comparatively nontoxic, after it has dissolved the polystyrene, it might be a slightly more dangerous, since any unreacted styrene monomer in solution is somewhat more toxic.

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  • $\begingroup$ Is it possible that the fresh juice from the lemon is causing a chemical reaction within the cup when used with a beverage? Or when the juice is added to a beverage, is the dilution enough to prevent a reaction? Cold tea/fresh lemon vs. hot tea/fresh lemon. $\endgroup$
    – JsuttraL
    Commented Mar 20, 2018 at 18:13
  • $\begingroup$ Are you saying that limonene will break down polystyrene? Or that traces of styrene from manufacturing would leech into solution? $\endgroup$
    – ralk912
    Commented Mar 20, 2018 at 18:49
  • $\begingroup$ I'm not sure about depolymerization, but unreacted monomer might be present, as well as various plasticizers, such as phthalates. Though high temperatures will depolymerize polystyrene, I have noted a strong styrene-like odor when the plastic is dissolved, or even shredded, at room temperature. $\endgroup$
    – DrMoishe Pippik
    Commented Mar 21, 2018 at 2:26
  • $\begingroup$ I did up voted too early as dissolving polystyrene, or anything, does not involve the breaking of covalent bonds. I doubt for the sudden release of monomer traces as they should be negligible or absent in a cup which is likely thought to contains drinks. Good point for limonene, tough. $\endgroup$
    – Alchimista
    Commented Mar 21, 2018 at 8:59

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